1. Field
Apparatuses and methods disclosed herein relate generally to an image photographing apparatus, and a controlling method thereof, and for example, to an image photographing apparatus configured to process an image and provide an out-focus effect, and a controlling method thereof.
2. Description of Related Art
When a user of an image photographing apparatus such as a camera, and so on, or a user of an electronic device equipped with image photographing function such as a mobile phone, and so on photographs an image, an out-of-focus subject of photography can be converged on an image sensor. In this case, the Bokeh can occur, in which the subject of photography is in defocus state. Here, the ‘Bokeh’ refers to a photographic effect in which an out-of-focus part is blurred, that is, to out-focus effect.
Generally, because the size of Bokeh is proportional to the square of the lens focal distance, image with sufficiently large Bokeh is infrequently acquired with a short focal distance lens. Accordingly, a device with a short focal distance lens, such as a mobile device camera, and so on, would generate a rather monotonous image in which the entire image regions are in focus, that is, without having out-focus effect.
In order to overcome the limits mentioned above, attempts are recently made to provide a mobile device or compact system camera (CSC) with a high-end digital single lens reflex (DSLR) effect, by applying a technology of providing artificial out-focus image through image post-processing.
A related method of post-processing an out-focus image achieves out-focus effect by extracting depth information during image acquisition through a camera, distinguishing a close subject and a far subject in the acquired image, and blur-filtering an image region of a certain subject in the image selected by the user.
However, while this related image post-processing method can be effective when the entire subjects have uniform brightness, it is not able to achieve sufficient desired effect for an image such as a night photography that includes a presence of light source or reflected light. The reason for this will be further described below with reference to FIG. 1.
FIG. 1A illustrates an example of an image that is photographed by a phone camera with a focal distance as short as approximately 4 mm. The photographed image 100 includes point light sources 102, 103 of a main subject 101. Because the lens focal distance of the phone camera is fairly short, a range of entire driving distance of the lens to adjust the focal point to the entire depths is approximately 0.5 mm. Accordingly, this means that it is very difficult to achieve Bokeh image effect by de-focusing of the point light sources 102, 103.
FIG. 1B illustrates an example of an image of the same object photographed in FIG. 1A, photographed with a DSLR camera equipped with a telephoto lens with a long focal distance and a small aperture value. Referring to the photographed image 110, the main subject 111 is in focus and thus expressed sharply, whereas the point light sources 112, 113 located in the far region are greatly de-focused and thus Bokeh image is generated. Because the total range of driving distance of the telephoto lens with long focal distance is several mm, when a focus is on the main subject in the near region, the subject in the far region is severely blurred. Accordingly, the point light sources 112, 113 in the far region generate large Bokeh, and this is how the Bokeh image that can be obtained in the high-end camera can be generated.
FIG. 1C illustrates an example of an image obtained as a result of distinguishing the image of FIG. 1A into a main subject and the far region, by out-focus post-processing the far region while keeping the main subject in the in-focus state. Referring to the out-focus post-processed image 100′, the main subject 101′ is expressed in focus, while most of the point light sources 102′, 103′ located in the background are disappeared.
Specifically, the point light sources 102, 103 of FIG. 1A are highly likely to be saturated within the pixels of the photographing element, since the point light sources 102, 103 have a considerably larger quantity of light compared to the adjacent background. That is, most pixels corresponding to the point light sources 102, 103 have maximum values. According to the related image post-processing method, in order to obtain out-focus effect, the output value of the pixels with maximum value is shared with the adjacent pixels, and according to this, the greater Bokeh effect is obtained by sharing many more output values to the adjacent pixels. Accordingly, after the blur filtering to obtain the Bokeh effect, only very weak signals are left, and thus almost all light sources 102′, 103′ disappear as illustrated in FIG. 1C.
FIG. 1D illustrates, when a pixel value (for example, brightness value) of the point light sources is 100%, a table which calculates a relation between a size of Bokeh (for example, radius) which may be generated through a blur filter according to the related art and brightness of Bokeh. As illustrated in FIG. 1D, when a size of Bokeh becomes 10 times greater, light amount substantially decreases to about 1%, and thus, when the Bokeh effect gets greater, the point light sources will eventually disappear.
That is, the simple blur filtering post-processing alone will not be able to render a Bokeh image of the point light sources 112, 113 that is as bright as in FIG. 1B.
As such, a technology is necessary, which can generate a Bokeh image appearing as natural as that photographed by a DSLR camera, by applying out-focus effect by post-processing an image photographed by a short focal distance lens, without having disappearance of the point light sources even with the increasing Bokeh size.